1. Field of the Invention
The present invention pertains to a slide-bead coating apparatus and method. More specifically, this invention pertains to a particular geometry used in the slide-bead coating apparatus for application of either a single flowing material or a plurality of flowing materials onto a moving substrate.
2. Description of the Related Art
Slide-bead coating is known in the art for supplying a flowing liquid layer or plurality of liquid layers down a slide surface to an efflux end, or lip, at which a liquid bridge, or bead, is formed in a gap between the lip and a moving substrate. As shown in FIG. 2 for coaters of prior art design, the liquid flow near the end of the slide surface has a profile shape that displays a thickened region followed by a thinning region followed by the liquid bridging the gap. The surface of the liquid flow at the end of the slide surface and in the gap is thus highly curved. The moving substrate carries away liquid from the liquid inventory in the bead in the same layered structure established on the slide. Exemplary examples are described in U.S. Pat. Nos. 2,761,791 and 2,761,419 issued, respectively, to Russell and Mercier et al.
For a given coater arrangement, substrate velocity, coating liquid and flow conditions, there is an operative range of applied differential pressures within which satisfactory coatings are obtained. The range limits are defined by the onset of bead instabilities and/or other practical considerations. As the differential pressure is increased above the operative range, or while maintaining the same differential pressure as the substrate velocity is increased above the associated operative range, the surface of the coating bead becomes so highly curved that the bead becomes unstable and gives rise to evenly-spaced disturbances, or ribbing, in the subsequent coating, as described by Saito et al. in "Instability of the Slide Coating Flow", 1982 Winter National AIChE Meeting, Orlando, Fla. If the differential pressure is decreased, a condition is reached whereby the differential pressure is insufficient to maintain an even covering of coating liquid over the desired width and/or the bead becomes unstable all along its width. The catastrophic results include narrowed and uneven coatings, or a complete loss of continuous coating. The difference in the limits of the upper extreme and the lower extreme for differential pressures described above constitutes what is herein called a useful differential pressure range for producing coatings of satisfactory quality and width. Examples of methods and apparatus to increase the useful differential pressure range are described in U.S. Pat. No. 4,443,504 issued to Burket et al., U.S. Pat. No. 3,996,885 issued to Jackson et al., U.S. Pat. No. 4,440,811 issued to Hitaka et al. U.S. Pat. No. 4,297,396 issued to Takehara et al. describes methods for increasing the maximum differential pressure, and U.S. Pat. No. 4,313,980 issued to Willemsens describes a method and device for reducing the minimum useful differential pressure.
It is known in the art that maximum differential pressure decreases as coating velocity increases. Therefore, increasing the maximum differential pressure at a given velocity is of utmost concern since this provides the practitioner with two choices, both of which are desirable. The coating velocity can remain fixed, in which case the increased maximum differential pressure provides operational latitude. Disturbances are less likely to cause perturbations in the solution flow. The increased maximum differential pressure also allows the practitioner to operate at a higher coating velocity, if desired, which has the expected benefits of higher productivity.
Improvements in the art are described in U.S. Pat. No. 3,993,019 issued to Jackson, wherein the slide comprises two regions. The region closest to the substrate is less downwardly inclined than the region further from the substrate, and of sufficient length to facilitate pooling just prior to the bead region. The pooling provides some advantage, yet further improvement in coating speed and operational latitude are still highly desired. The advantage is best observed when the coating is applied near the horizontal centerline of the roll, and decreases as other coating configurations are utilized. However, with pooling, particles may more readily settle from the coating liquid, become attached to the lip and produce disturbances in the liquid flow, thus resulting in defects in the subsequent coating.
Upwardly directed flow designs are taught in U.S. Pat. Nos. 4,283,443 and 4,299,188 issued, respectively, to Choinski and Isayama et al. The upwardly directed lip region in U.S. Pat. No. 4,283,443 is required to be of sufficient length as to result in pooling. Consequently, the operational latitude advantage is limited, and it is susceptible to the deleterious settling effect. The upwardly directed lip region of U.S. Pat. No. 4,299,188 is limited in size, and results in an incrementally improved operational latitude advantage. However, a practical realization of this design will still be susceptible to pooling although to a lesser extent. Furthermore, both of these upturned lip designs provide a lip edge which is sharp and easily damaged. A damaged lip edge is deleterious to coating quality, and replacement or repair time causes lost productivity.
It is an object of the present invention to provide a slide-bead coating apparatus which increases the maximum differential pressure for onset of coating ribbing. It is another object of the present invention to provide a slide-bead coating apparatus which can be operated at a higher coating velocity. It is another object of the present invention to provide a slide-bead coating apparatus which can provide increased yields.